Arctic offshore regions are viewed as the last frontier of oil and gas mega project development. There are vast untapped oil-bearing regions lying from the Canadian and American Beaufort Seas around the Arctic Circle through to the oil-prone regions of Russia. The areas have seen a lot of activity, but little or no offshore production has been realized. The lack of development has been caused primarily by the capital expenditure required to achieve production versus the currently depressed oil prices. Hence any development in the Arctic has continued to be uneconomic. However given the vast deposits in the Arctic, it is worthwhile developing. In view of the manner in which conventional offshore drilling and production systems have adapted to the lower oil prices, it is timely to develop an economic approach to the exploration, drilling and production of offshore oil wells in the Arctic.
Three approaches used in the non-Arctic areas of offshore drilling and production are of interest with respect to rendering such projects economic. These approaches are in the form of subsea templates, tender assisted drilling (TAD) and jack-up drilled wellhead platforms. Subsea template concepts work reasonably well in offshore areas; however, would not be acceptable in the Arctic due to potential problems with ice scour on the sea floor and the consequences of ice impact on the template. Furthermore, this approach is not in step with the very limited drilling season in the Arctic which may be in the range of 100 to 120 days. In this period at best, one might drill and complete two shallow wells where, in actual fact, some twenty to forty wells might be required to develop a field.
Tender assisted drilling overcomes the problems associated with drilling equipment taking up space on production platforms. However, these systems are not suitable for use in the Arctic since they are very ice crush sensitive.
Jack-up drilled wellhead platforms, which are a version of a tender assisted drilling, are suitable for shallow water depths and benign environmental regions. Although the systems are economic, they are ice crush sensitive and would not be suitable for unprotected use in the Arctic.
Attempts to achieve year-around oil well drilling and production have to some extent been satisfied by five basic systems which are either no longer in use or have limited use:
1. ice islands; PA1 2. sacrificial beach islands; PA1 3. ice strengthened drill ships with ice class support; PA1 4. shallow caisson retained islands; and PA1 5. deep mobile Arctic caissons. PA1 1. achievement of rapid, first oil production date; PA1 2. ease of construction and installation and hook-up; PA1 3. use of converted mobile offshore drilling units; PA1 4. separation of drilling and production functions as required; PA1 5. potential use of tie-back wells; PA1 6. provides for twin drilling facilities not possible on other mobile caissons with the additional advantage of relief well drilling capability; PA1 7. minimizes impact of the drilling facilities on the platform design; PA1 8. is capable of a multi-well drilling through separated well bases, i.e. multi-slot drilling; PA1 9. a self-sufficient system; PA1 10. provides for export of recovered oil; and PA1 11. can be mobilized and moved to any desired region in the Arctic. PA1 i) a floatable generally four-sided caisson with a fixed upstanding front outer wall and fixed upstanding opposing side outer walls and an upstanding rear outer wall having a removable entrance door, PA1 ii) the caisson being of a height to permit offshore drilling and/or production when the caisson is submersed to rest on an ocean floor, PA1 iii) the outer walls being reinforced to withstand crushing forces exerted by ice bearing offshore oceans and the outer wall height being sufficient to turn back a maximum design wave, the outer walls having an inwardly sloping wall portion about caisson perimeter above and below submersed water level to wedge upwardly advancing ice, PA1 iv) the upstanding front and rear walls and the opposing side walls define a cavity, access to which is provided by removing the entrance door, PA1 v) the cavity has a base, a front inner wall, opposing side inner walls and a rear inner wall on the entrance door, the inner walls being spaced from corresponding outer walls of the front and opposing sides with an upper support deck spanning the spaced apart outer and inner walls, the front wall and rear wall including the door and the side inner and outer walls define a caisson perimeter structure, the interior space of the cavity being protected from ice crushing forces by the caisson perimeter structure, PA1 vi) the perimeter structure and the base have a plurality of ballast compartments to float the caisson and when the compartments are filled with a ballast liquid, the caisson sinks towards and rests on the ocean floor where the caisson perimeter structure is of a height to provide above water level a sea break to safeguard operating facilities and personnel, PA1 vii) the base of the cavity is positioned relative to the deck to provide sufficient draft when the caisson is floating and the door is open to permit entrance and exit of mobile offshore drilling units and/or production units, PA1 viii) the deck along each of the opposing sides has at least one drilling/production service hatch, PA1 ix) each of the side portions of the perimeter structure has extending vertically therethrough, a drilling/production moon pool system which extends from and is in communication with the corresponding service hatch, PA1 x) the caisson perimeter structure comprises an ocean floor perimeter foot portion of solid reinforced structural material, the cavity base being of reinforced structural material tied into the perimeter foot portion, PA1 xi) the inner and outer walls to at least above submersed water level when the perimeter foot is resting on ocean floor are of reinforced structural material, the plurality of the ballast compartments being provided between the inner and outer walls with a majority of each ballast compartment being below submersed water level when the perimeter foot is resting on ocean floor.
Ice islands have very limited application, certainly in waters no deeper than fifteen meters. A common problem with ice islands is their tendency to break-up and hence problems in maintaining the stability of the ice platform through summer months. Sacrificial beach islands are possible in water depths of fifteen to twenty meters. Such islands are very expensive to construct and do not lend themselves to any form of mobility as the need might arise to move the drilling and/or production equipment. Furthermore, the shallow slope of the island construction leads to problems with wave run-up and a continual need to maintain the shore of the island. Ice strengthened drill ships are possible; however, such ships are dedicated to either full scale Arctic drilling or production development. Such units are almost solely dedicated to exploration or production drilling and have no practical application as a production unit. Shallow caisson retained islands are another possible solution, like a sacrificial beach island are subject to potential wave erosion. They also do not lend themselves well to efficient topside design and, as such, have limited application.
Deep mobile Arctic caissons have become accepted to some extent in drilling and/or production in the Arctic. Such units have the ability to operate in deeper water depths even up to forty to fifty meters, but preferably in the range of twenty meters. Such units are very large and usually combine drilling with a limited production testing capability. However, the space taken up by the drilling systems is constant and can represent some 50% of the available deck area so that, once drilling is completed, this space is wasted for production purposes.
Examples of these types of units are found in the prior art. Ice structures are disclosed in U.S. Pat. No. 4,699,545. The ice island is developed by spraying water. However considering the size of the ice island to be developed, this can be a time-consuming program with a limited winteruse only application. Another technique for developing large bodies of ice, which may be used in offshore drilling, is disclosed in U.S. Pat. No. 4,431,346. U.S. Pat. No. 4,596,291 discloses a floating, semi-submersible offshore drilling platform which is dedicated to the drilling of oil wells. The drilling unit is to some extent movable on the platform for drilling from various positions. The system is protected against ice by mounting the platform on a submersible pontoon which may be formed of concrete. When winter sets in, the system is elevated where the concrete portion of the system resists impact of ice. In order to protect the drilling system during the winter months, the drilling may take place through one of the columns which is connected to the pontoon portion, hence protecting the drill system from ice in the winter months. It is apparent, however, that this system would have limited use in heavy ice areas, such as in the Arctic. Another form of mobile semi-submersible caisson for use in oil well drilling is disclosed in U.S. Pat. No. 5,098,219. This system involves the sinking of a caisson beneath the water where the drilling system is contained within the caisson. The crew is always within the caisson where they move to and from their quarters by way of an elevator. This is, of course, not conducive to long term drilling operations, because of the submerged nature of the system.
A semi-submersible oil production system is disclosed in U.K. patent application 2,185,446. The system is in the form of an octagon which provides a strong-box structure enclosing a number of production decks. However, this system is tied down by cables and hence is not particularly suitable for use in the Arctic. Little or no consideration to ice resistance has been given in the design of the underwater pontoon hull.
Floatable caissons, which are capable of being sunk to rest on the ocean floor and which are suitable for use in the Arctic, are disclosed in Russian patent 1,700,138 and Canadian patent 1,178,812. The Russian design is circular in shape, reinforced about its perimeter and filled with fill material to stabilize the ballasted structure. Similarly, the caisson of the Canadian patent is ballasted to rest on the ocean floor and then the central region thereof filled with fill material which is kept in an unfrozen state. The perimeter of the caisson defines a support on which an upper working deck is constructed. Drilling or production can take place from the working deck. The perimeter of the caisson is designed to have a sloping surface which causes advancing ice to wedge upwardly, break and fall away from the caisson. The caisson, when in position, does permit year round drilling and at any time when the Arctic is reasonably free of ice cover, can be floated and moved to another location. However during its use, the systme is either normally dedicated to drilling production or both drilling and production with the consequent increase in capital expenditure.
To some extent the demands in oil well drilling and production have been met by the prior art, particularly the submersible caisson of Canadian patent 1,178,812. However, the capital expenditure associated with this type of caisson design requiring its own form of oil well drilling and/or oil well production unit, is far too high hence rendering its use uneconomic in the Arctic for purposes of oil well production.
According to this invention, a submersible mobile caisson is provided which addresses these problems and allows the use of readily available, inexpensive, mobile offshore drilling units and mobile offshore production units which, in their normal habitat, are ice crush sensitive and could not, without the caisson of this invention, be used in the Arctic. The mobile caisson of this invention accomplishes these features without the normally prohibitive capital expenditure. The mobile caisson of this invention provides for: